In the anatomy of the spine, as shown in FIG. 1, the sacrum is situated beneath the lumbar vertebrae and is made up of five vertebrae written S1 to S5 which, over the course of human evolution, have become fused to one another. Nowadays, four pairs of holes, referred to as intervertebral foramens 11, remain between the sacral vertebrae S1 to S5. These holes, distributed on either side of the sagittal plane of the spine constitute traces of the epoch when these vertebrae were separate.
The top vertebra of the sacrum, written S1, is articulated to the fifth lumbar vertebra, written L5, as shown in FIG. 1. This joint constitutes the lumbo-sacral joint, or the L5-S1 joint.
Furthermore, each lumbar vertebra presents a middle posterior projection known as the spinous process 10, sometimes referred to below as the process. The sacral vertebrae have lost their spinous processes during evolution, and instead they merely conserve respective small residual bumps 12.